Electrolyte for electrochemical battery cell and battery cell containing the electrolyte

ABSTRACT

Electrolyte for an electrochemical battery cell, containing sulfur dioxide and a conductive salt. Improved characteristics of a cell filled with the electrolyte are achieved in that the molar concentration of hydroxide groups in the electrolyte is at most 50 mmol per liter and the molar concentration of chlorosulfonate groups in the electrolyte is at most 350 mmol per liter.

RELATED APPLICATIONS

This application is a continuation of PCT/EP2013/000366, filed Feb. 7,2013, the entire disclosure of which is hereby incorporated by referencein its entirety.

BACKGROUND

The invention concerns an electrolyte for an electrochemical batterycell. The electrolyte contains sulfur dioxide and a conductive salt. Theinvention also refers to a process for manufacturing the electrolyte anda battery cell containing the electrolyte.

Rechargeable battery cells are of great importance in many technicalfields. Development goals are in particular a high energy density(charge capacity per unit of weight and volume), a high charging anddischarging current (low internal resistance), a long service life witha large number of charging and discharging cycles, very good operatingsafety and the lowest possible costs.

The electrolyte is an important functional element of every batterycell. It contains a conductive salt and is in contact with the positiveand negative electrodes of the battery cell. At least one ion of theconductive salt (anion or cation) has such mobility in the electrolytethat the charge transport between the electrodes, which is required forfunctioning of the cell, can take place by ion conduction.

SUMMARY

An SO₂-based electrolyte is used according to the invention. In thecontext of the invention, this term designates an electrolyte containingsulfur dioxide not just in low concentration as an additive, but inwhich the SO₂ at least to some degree enables the mobility of the ionsof the conductive salt contained in the electrolyte, thus ensuring thecharge transport. The electrolyte preferably contains at least 20percent by weight (“wt. %”) SO₂, values of 35 wt. % SO₂, 45 wt. % SO₂and 55 wt. % SO₂, relative to the overall quantity of the electrolytecontained in the cell, being further preferred in this order. Theelectrolyte can also contain up to 95 wt. % SO₂, maximum values of 85wt. % and 75 wt. % [obvious error in German text: “75 wt. % and 85 wt.%”] being preferred in this order.

The electrolyte is preferably used in an alkali metal cell where theactive metal is an alkali metal. However, the active metal may also bean alkaline earth metal or a metal from the second subgroup of theperiodic table. The term active metal designates the metal whose ionsmigrate to the negative or positive electrode within the electrolyteduring charging or discharging of the cell and participate there inelectrochemical processes that lead directly or indirectly to thetransfer of electrons into or out of the external circuit. The activemetal is preferably lithium, sodium, calcium, zinc or aluminum, lithiumbeing particularly preferred. Lithium cells with an SO₂-basedelectrolyte are designated as Li—SO₂ cells. By way of example (butwithout limiting the generality), reference will be made hereafter tolithium as the active metal of the negative electrode.

In the case of an alkali metal cell, a tetrahalogenoaluminate ispreferably used as the conductive salt, particularly preferably atetrachloroaluminate of the alkali metal, such as LiAlCl₄. Furtherpreferred conductive salts are aluminates, halogenides, oxalates,borates, phosphates, arsenates and gallates of an alkali metal, inparticular of lithium.

Since many years there have been discussions about SO₂-basedelectrolytes for lithium cells. In

-   D1 “Handbook of Batteries”, David Linden (Editor), 2nd edition,    McGraw-Hill, 1994    the high ionic conductivity of an SO₂-based inorganic electrolyte is    emphasized. It is stated that this electrolyte is also advantageous    with respect to other electrical data. It is further stated therein    that systems with an SO₂-based electrolyte have been under    investigation for a long time and are of interest for special    applications, but that the further commercial applicability is    restricted, in particular since the electrolyte is highly corrosive.

An advantage of the SO₂-based electrolyte is that—in contrast to theorganic electrolytes of the lithium-ion cells common in practice—itcannot burn. The known safety risks of lithium-ion cells are mainlycaused by their organic electrolytes. If a lithium-ion cell catches fireor even explodes, the organic solvent of the electrolyte forms thecombustible material. An electrolyte according to the invention ispreferably essentially free of organic materials, whereby “essentiallyfree” is to be construed such that the quantity of any organic materialspresent is so small that they do not represent any safety risk.

On this basis, the invention addresses the technical problem of makingavailable an SO₂-based electrolyte which—while maintaining theadvantageous characteristics of such electrolytes—leads to improvedelectrical characteristics of an electrochemical battery cell filledwith the electrolyte.

The problem is solved by an electrolyte according to claim 1. In theelectrolyte, the content of compounds containing a hydroxide group (OH⁻)is so low that the molar concentration of hydroxide groups in theelectrolyte is at most 50 mmol (millimol) per liter. At the same time,the content of compounds containing a chlorosulfonate group (SO₃Cl⁻) isso low that the molar concentration of chlorosulfonate groups in theelectrolyte is at most 350 mmol per liter.

An SO₂-based electrolyte is usually produced by mixing the Lewis acidcomponent and Lewis base component of the conductive salt with eachother and allowing them to react with gaseous SO₂ that is allowed toflow over or through the mixture. In an exothermic reaction, a Lewisacid/Lewis base adduct is formed which is dissolved in SO₂, e.g.:LiCl+AlCl₃→LiAlCl₄. When the conductive salt dissolves in SO₂ its ionsbecome mobile, e.g. Li⁺ and AlCl₄ ⁻.

This process is described in the literature, for example in

-   D2 U.S. Pat. No. 4,891,281 and-   D3 D. L. Foster et al: “New Highly Conductive Inorganic    Electrolytes”, J. Electrochem. Soc., 1988, 2682-2686.

A problem that has already been discussed for a long time is that duringproduction of the electrolyte, traces of water are dragged in whichreact to produce hydrolysis products, said hydrolysis productscontaining hydroxide groups. The following reaction takes place, forexample:H₂O+LiAlCl₄→AlCl₃OH⁻+Li⁺+HCl  (A)

The following publications address this problem:

-   D4 U.S. Pat. No. 4,925,753    -   In the cell described here, the SO₂ serves both as a solvent of        the conductive salt and as a liquid cathode. The document        describes how moisture and hydrolysis products are dragged into        the electrolyte by the starting materials and lead to increased        corrosion of the cell components, in particular the lithium        anode. In order to avoid moisture being dragged in, one Lewis        component (alkali metal salt) is dried at 200 degrees Celsius        for 16 hours and the other Lewis component (aluminum chloride)        is freshly sublimated. In addition, the concentration of        aluminum is increased (e.g. by increasing the concentration of        LiAlCl₄) in order to achieve a higher starting capacity during        operation of the cell. A calcium salt of the same anion is added        additionally which serves as an “anti-freeze agent”,        compensating an increase in the freezing temperature of the        electrolyte caused by the increased concentration of LiAlCl₄.-   D5 U.S. Pat. No. 5,145,755    -   This document describes the study of an electrolyte produced        according to D 4 by means of IR spectral analysis. This shows a        strong and wide absorption band in the area of the OH        oscillation. The cleaning effect of the process described in D4        is thus insufficient. A different method for removing hydrolysis        products from the electrolyte solution is described in D5. Here,        the starting salts (Lewis acid and Lewis base) are mixed and        heated with sulfuryl chloride under reflux to 90° C. The salt        mixture is then melted to 120° C. to 150° C. to remove the        sulfuryl chloride. By feeding SO₂ gas to the salt mixture, an        electrolyte is produced that is said to be essentially free of        hydrolysis products.-   D6 I. R. Hill and R. J. Dore: “Dehydroxylation of LiAlCl₄.xSO₂    Electrolytes Using Chlorine”, J. Electrochem. Soc., 1996, 3585-3590    -   This publication describes as an introduction the previous        attempts of dehydroxylation of SO₂-based electrolytes. It is        explained that a significant disadvantage of this electrolyte        type is that it normally contains hydroxide contamination and        that the previous attempts to eliminate this contamination were        insufficient. On the basis of the fact that the required        dehydroxylation cannot be achieved by heating, the authors        conclude that chemical treatment is required. With respect to        the dehydroxylation by means of sulfuryl chloride described in        D5, they criticize the fact that recontamination with water can        occur when the electrolyte is produced using the cleaned salt.        For this reason, they say that dehydroxylation of the        LiAlCl₄.xSO₂ electrolyte should be preferred. To this end the        document compares two processes where the electrolyte is treated        with sulfuryl chloride (SO₂Cl₂) and chlorine gas (Cl₂)        respectively. It is stated that both processes allow sufficient        dehydroxylation. The chlorine gas method is seen as the        preferred method. As shown in the IR spectra in the document,        chlorosulfonate groups are produced in both processes which        replace the hydroxide groups. The electrochemical activity of        the chlorosulfonate groups is investigated by observing the        intensity of the corresponding infrared bands during extensive        discharge of the cell. It is stated that the intensity of the        bands does not decrease and that consequently the        chlorosulfonate groups do not participate in the cell reactions.

In the context of the invention, it was established that (SO₃Cl)⁻ whichis inevitably produced in the known processes for removal of compoundscontaining hydroxides, significantly impairs functioning of the cell andthat a considerable improvement, in particular with respect to thecharging capacity of the cell and its usability for a large number ofcharging and discharging cycles, is achieved if not only the molarconcentration (also designated as mole number) of hydroxide groups inthe electrolyte is below 50 mmol per liter, but simultaneously the molarconcentration of chlorosulfonate groups in the electrolyte does notexceed a maximum value of 350 mmol per liter. Particularly good resultsare achieved if the molar concentration of hydroxide groups in theelectrolyte is at most 45 mmol per liter, preferably at most 25 mmol perliter, further preferably at most 15 mmol per liter and particularlypreferably at most 5 mmol per liter. With respect to the molarconcentration of chlorosulfonate groups in the electrolyte, it isparticularly advantageous if its maximum value does not exceed 250 mmolper liter, preferably 200 mmol per liter and particularly preferably 150mmol per liter.

As already described, hydroxide groups can be produced by water tracesbeing dragged into the starting materials for electrolyte production orinto the electrolyte itself. According to reaction equation (A), thewater can react with the electrolyte to produce the hydroxide-containingcompound AlCl₃OH⁻. However, other hydroxide-containing compounds canalso be produced. All hydroxide-containing compounds can be detectedusing infrared spectroscopy by way of the OH oscillation at a wavenumberof around 3350 cm⁻¹. In contrast to infrared spectroscopy, the knownKarl Fischer method for analysis of water traces is not suitable fordetermination of hydroxide-containing compounds in the electrolyte. Inaddition to hydroxide-containing compounds such as AlCl₃OH⁻, the KarlFischer method also detects oxide-containing compounds of theelectrolyte such as AlOCl. A high Karl Fischer value therefore does notcorrespond to a high concentration of hydroxide-containing compounds.

Compounds containing chlorosulfonate groups are produced, for example,by the reaction of chlorine with hydroxide-containing compounds of theelectrolyte solution according toAlCl₃OH⁻+Cl₂+SO₂→AlCl₃(SO₃Cl)⁻+HCl  (B)

Compounds containing chlorosulfonate groups can be detected in theelectrolyte by means of infrared spectroscopy. Three bands atwavenumbers of approximately 665 cm⁻¹, 1070 cm⁻¹ and 1215 cm⁻¹ arecharacteristic for the presence of compounds with chlorosulfonategroups.

The preferred percentages by weight of SO₂ in the overall quantity ofthe electrolyte contained in the cell have already been stated. Thepercentage by weight of the conductive salt in the electrolyte shouldpreferably be less than 70%, values of less than 60, 50, 40, 30, 20 and10 wt. % being further preferred in this order.

The electrolyte should preferably comprise mainly the SO₂ and theconductive salt. The percentage by weight of SO₂ plus conductive saltreferred to the overall weight of the electrolyte in the cell shouldpreferably be more than 50 wt. %, values of more than 60, 70, 80, 85,90, 95 and 99% being further preferred in this order.

Several different salts may be dissolved in the electrolyte such that atleast one of their ions is mobile in the electrolyte and contributes byion conduction to the charge transport required for functioning of thecell, so that the salt acts as a conductive salt. The fraction of saltswhose cation is the cation of the active metal preferably predominates.Referred to the mole number of all salts dissolved in the electrolyte,the mole fraction of dissolved salts with a cation different from thecation of the active metal in the electrolyte should be at most 30 mol%, values of at most 20 mol %, 10 mol %, 5 mol % and 1 mol % beingfurther preferred in this order.

With respect to the molar relation of conductive salt and sulfurdioxide, it is preferred that the electrolyte contains at least 1 moleof SO₂ per mole of conductive salt, with values of 2, 3, 4 and 6 molesof SO₂ per mole of conductive salt being further preferred in thisorder. Very high molar fractions of SO₂ are possible. The preferredupper limit can be specified as 50 moles of SO₂ per mole of conductivesalt and upper limits of 25 and 10 moles of SO₂ per mole of conductivesalt are further preferred in this order.

As explained above, the electrolyte according to the invention ispreferably essentially free of organic materials. However, this does notexclude some embodiments of the invention also containing organicmaterials in the electrolyte, such as one or a plurality of organicco-solvents. In such an embodiment, however, the overall quantity of theorganic material in the electrolyte should in any case be less than 50wt. %, with values of less than 40, 30, 20, 15, 10, 5, 1 and 0.5 wt. %,relative to the total weight of the electrolyte, being further preferredin this order. According to a further preferred embodiment, the organicmaterial has a flash point of less than 200° C., with values of 150,100, 50, 25 and 10° C. being further preferred in this order.

According to a further preferred embodiment, the electrolyte containstwo or more organic materials, the organic materials having an average(calculated from the weight ratio) flash point of less than 200° C.,values of 150, 100, 50, and 10° C. being further preferred in thisorder.

A process suitable for production of an electrolyte according to theinvention is characterized by the following steps:

-   -   A Lewis acid, a Lewis base and aluminum are mixed in solid form.    -   The mixture is kept at a minimum temperature for a minimum        period of 6 hours, the minimum temperature being above the        melting point of the mixture and at least 200° C. An adduct of        the Lewis acid and the Lewis base is formed.

The minimum temperature is preferably 250° C., values of 300° C., 350°C., 400° C., 450° C. and 500° C. being particularly preferred in thisorder. The minimum period is preferably 12 hours, values of 18, 24, 48and 72 being particularly preferred in this order.

The fraction of aluminum in the starting mixture should be at least 40mmol aluminum per mole of the Lewis acid, values of 200 and 400 mmol permole of Lewis acid being further preferred in this order.

The Lewis acid is preferably AlCl₃. The Lewis base is preferably achloride of the conductive salt, thus LiCl in the case of a lithiumcell.

The starting substances are preferably used in particle form and wellmixed before heating. The increase in temperature should take placeslowly, mainly to avoid a rapid increase in pressure. In order tocompensate for a possible increase in the gas pressure, the reactionvessel should be open at least at the start of the heating process,undesired ingress of external gases being favorably prevented byapplication of a vacuum or use of a liquid seal similar to a washbottle. It may be favorable to remove solid contamination, in particularaluminum, by filtration (e.g. using a fiber glass filter cloth) at theend of the process. Filtration should take place at a temperature wherethe melt is sufficiently liquid to pass through the filter. On the otherhand, the temperature should be low enough to avoid damage to the filterand any contamination of the melt caused thereby. A temperature of 250°C. has proven to be suitable in practice.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail hereafter with reference tofigures, exemplary embodiments and experimental results. The featuresdescribed can be used individually or in combination to create preferredembodiments of the invention. In the figures:

FIG. 1 shows a cross-sectional view of a battery cell according to theinvention;

FIG. 2 shows FTIR spectra (transmission) of calibration electrolytesolutions with five different molar concentrations of hydroxide groups;

FIG. 3 shows FTIR spectra (transmission) of electrolytes with differentmolar concentrations of hydroxide groups;

FIG. 4 shows a graphic representation of the dependence of the number ofcycles at which a discharge capacity of 66.5% of the nominal capacity isreached for cells which contain different molar concentrations ofhydroxide groups;

FIG. 5 shows a graphic representation of the capacity irreversiblyconsumed for formation of the covering layer on the electrodes for cellswith different molar concentrations of hydroxide groups;

FIG. 6 shows a graphic representation of the discharge capacity as afunction of the number of cycles of two cells with different molarconcentrations of hydroxide groups in the electrolyte;

FIG. 7 shows an FTIR spectrum (ATR) of two electrolytes that containdifferent molar concentrations of chlorosulfonate groups;

FIG. 8 shows a graphic representation of the relationship of thecovering layer capacity and the discharge capacity for cells withdifferent molar concentrations of chlorosulfonate groups in theirelectrolyte.

DESCRIPTION

The housing 1 of the rechargeable battery cell 2 shown in FIG. 1encloses an electrode arrangement comprising a plurality (three in thecase shown) of positive electrodes 4 and a plurality (four in the caseshown) of negative electrodes 5. The electrodes 4, 5 are connected inthe usual manner with corresponding terminal contacts 9, 10 of thebattery by means of electrode leads 6, 7. The cell is filled with anSO₂-based electrolyte 8 in such a manner that the electrolyte preferablypenetrates completely into all pores, in particular inside theelectrodes 4, 5. The electrolyte can be in liquid or gel form.

As is common, the electrodes 4, 5 have a planar shape, i.e. they areshaped as layers having a thickness which is small relative to theirextension in the other two dimensions. The electrodes 4, 5 comprise inusual manner a current collector element which is made of metal andserves to provide the required electronically conductive connection ofthe active material of the respective electrode. The current collectorelement is in contact with the active material involved in the electrodereaction of the respective electrode. The electrodes are separated fromeach other by separators 11 in each case. The housing 1 of the prismaticcell shown is essentially cuboid, the electrodes and the walls shown incross-section in FIG. 1 extending perpendicularly to the drawing planeand being essentially straight and flat. However, the cell according tothe invention can also be designed as a spirally wound cell.

The negative electrodes 5 are preferably insertion electrodes, i.e.comprise an electrode material in which the ions of the active metal areinserted during charging of the cell and from which they are extractedduring cell discharge. Preferably the negative electrodes containcarbon.

The active mass of the positive electrode is a component of the cellwhich changes its charge state as a result of the redox reaction thattakes place at the positive electrode. In the cells according to theinvention, the active mass of the positive electrode is preferably anintercalation compound into which the active metal can be inserted.Metal compounds are especially suitable (e.g. oxides, halogenides,phosphates, sulfides, chalcogenides, selenides), compounds of atransition metal being especially suitable, in particular an element ofthe atomic numbers 22 to 28, especially cobalt, nickel, manganese oriron, including mixed oxides and other mixed compounds of the metals.Lithium iron phosphate is particularly preferred. When such a cell isdischarged, ions of the active metal are inserted in the positive activemass. For reasons of charge neutrality, this leads to an electrodereaction of the positive active mass at the electrode where an electronis transferred from a current collector element of the electrode to thepositive active mass. The reverse process takes place during charging:the active metal (e.g. lithium) is extracted as an ion from the positiveactive mass and an electron is transferred from the latter to thecurrent collector element of the positive electrode.

FIGS. 2 to 8 are based on the experimental testing of the invention.

FIG. 2 shows FTIR spectra of calibration solutions with different molarconcentrations of hydroxide groups. The absorbance A is shown as afunction of the wavenumber k.

Suitable calibration solutions can be produced, for example, by adding adefined quantity of lithium chloride monohydrate to an electrolyte thatdoes not show any OH absorption band, i.e. does not contain anyhydroxide groups. Addition of 0.0604 g lithium chloride monohydrateincreases the water content, and thus also the hydroxide group contentof the calibration electrolyte, by 1 mmol.

Calibration electrolytes with different molar concentrations ofhydroxide groups were analyzed by means of FTIR spectroscopy in therange of the absorption band of OH⁻ (3300 cm⁻¹). FIG. 2 shows thespectra for the five molar concentrations of hydroxide groups stated inthe graph.

FIG. 3 shows a representation corresponding to FIG. 2 wherein, inaddition to the calibration curves for the molar hydroxideconcentrations zero (dotted) and 76 mmol per liter (continuous line),the FTIR spectrum of an electrolyte is shown (dashed line) that wasproduced in accordance with the instructions of the document D3 citedabove. The spectrum shows that the electrolyte produced according tothis state of the art contained approximately 94 mmol per liter(corresponding to approx. 1000 ppm) of hydroxide groups. The above citeddocument D6 also states that an uncleaned electrolyte contains ahydroxide amount corresponding to this molar concentration.

Hydroxide-containing compounds have a detrimental effect on theelectrochemical properties of a battery cell. The discharge capacityQ_(D) specifies the charge capacity which can be extracted from abattery cell during discharge. Generally, Q_(D) decreases from cycle tocycle during charging and discharging. The smaller this decrease, thelonger is the service life of the battery.

FIG. 4 shows the influence of the molar concentration of hydroxidegroups on the decrease in capacity and thus on the service life of thebattery cell. The graph is based on an experiment where battery cellswith two negative carbon electrodes, an SO₂-based electrolyte withLiAlCl₄ as conductive salt and a positive electrode with lithium ironphosphate are charged and discharged over several hundred cycles. Thenominal capacity of the cell was 100 mAh. Charging of the cells tookplace with 1 C, corresponding to a current of 100 mA up to anend-of-charge voltage 3.6 V and a drop in the charging current to 40 mA.After this, the cells were discharged with the same current until apotential of 2.5 V was reached. There was a pause of ten minutes in eachcase between charging and discharging.

FIG. 4 shows the number of charging and discharging cycles performedwith the test cells until a defined minimum capacity (here 66.5% of thenominal capacity) was reached. The hydroxide-free cell, which isrepresented by the left column, reached this value only after 500cycles. In contrast, the other cells with a hydroxide content of 16[obvious error in German text: “19”], 40 and 50 mmol/l achieved muchlower numbers of cycles, the cell with a hydroxide content of 50 mmol/lachieving only approx. 300 cycles. Assuming, for example, that a batterycell is charged and discharged once daily and is to be used up to thespecified discharge capacity, this means that the hydroxide-free cellhas a service life of 1 year and 7 months, whereas the cell with ahydroxide content of 50 mmol/l can be used only for a period of 10months.

As already explained, hydroxide groups contained in the electrolyte ofan electrochemical cell lead to a deterioration in the electrical dataof said cell in so far as the charge quantity irreversibly consumed inthe initial charging cycles for formation of an electrode covering layer(“covering layer capacity” Q_(C)) increases as a function of the molarconcentration of hydroxide ions. The covering layer capacity Q_(C) canbe determined, for example, by comparing the charge and dischargecapacities of the cell in the first cycle. FIG. 5 shows the results ofsuch experiments. The covering layer capacity Q_(C) (as a percentage ofthe theoretical charge capacity of the negative electrode) is shown as afunction of the molar concentration M of hydroxide ions contained infour different electrolytes. It can be seen that the covering layercapacity is higher for a cell with 50 mmol/l than for a cell whoseelectrolyte does not contain any hydroxide ions. The useful dischargecapacity of cells that do not contain any hydroxide is correspondinglyhigher.

The effect is substantial since all following charging and dischargingcycles for a hydroxide-containing cell start at a correspondingly lowerlevel than with hydroxide-free cells. FIG. 6 shows the dischargecapacity Q_(D) as a percentage of the nominal capacity as a function ofthe number of charging and discharging cycles, the continuous curveshowing the results with a hydroxide-free electrolyte and the dashedcurve the results for an electrolyte with a molar concentration ofhydroxide groups of 50 mmol/l.

As described above, different methods were tested in the past in orderto remove hydroxide-containing contamination of the electrolyte and thuseliminate the associated disadvantages. It was established that thedesired cleaning effect cannot be achieved by use of dried startingsubstances and/or heating the electrolyte. For this reason, chemicalmethods using chlorine or chlorine-containing substances were proposed(cf. D5 and D6). However, it was established in the context of theinvention that the formation of chlorosulfonate groups in theelectrolyte associated with such methods causes additional problems.

FIG. 7 shows the FTIR spectrum (ATR), namely the absorbance A as afunction of the wavenumber k, for two electrolyte solutions thatcontained no (dashed line) sulfonate groups and 290 mmol/l (continuousline) of sulfonate groups respectively. Three bands can be clearly seenat the wavenumbers 665 cm⁻¹, 1070 cm⁻¹ and 1215 cm⁻¹ which occur due tothe presence of compounds containing chlorosulfonate groups.

FIG. 8 shows the covering layer capacity Q_(C) for cells whoseelectrolyte contained three different molar concentrations ofchlorosulfonate groups. These measurements were performed as half-cellexperiments in a three-electrode system (working electrode: carbon(graphite); counter electrode: lithium; reference electrode forcurrentless potential measurement: lithium). The electrodes were placedin a glass E-cell and filled with the electrolyte solution to beexamined in each case. The left column shows the example of a cell withan electrolyte according to the invention, which was essentially free ofhydroxide groups, but was simultaneously essentially free ofchlorosulfonate groups. The covering layer capacity is only 17% here.The two other columns show the results for cells with 73 mmol/l and 291mmol/l of chlorosulfonate groups. The higher the covering layercapacity, the lower is the discharge capacity. This means that thepercentage relationship between the (irreversible and thus wasted)covering layer capacity Q_(C) and the useful discharge capacity Q_(D) issignificantly worsened due to the chlorosulfonate content.

An electrolyte according to the invention can be produced, for example,by means of the following process:

-   a) Drying: Lithium chloride is dried under vacuum for three days at    120° C. Aluminum particles are dried under vacuum for two days at    450° C.-   b) Mixing: 434 g (10.3 mol) LiCl, 1300 g (9.7 mol) AlCl₃ and 100 g    (3.4 mol) Al are mixed well in a glass bottle with an opening that    allows gas to escape. The quantities correspond to a mole ratio    AlCl₃:LiCl:Al of 1:1.06:0.35.-   c) Melting/heat treatment: The mixture is heat-treated as follows:    -   Two hours at 250° C.;    -   two hours at 350° C.;    -   two hours at 500° C.;    -   after 6 hours the opening of the bottle is closed;    -   three days at 500° C.;-   d) Cooling/filtering: After cooling to 250° C., the melt is filtered    through a fiber glass cloth.-   e) Addition of SO₂: The melt is cooled to room temperature after one    day. The bottle with the melt is evacuated. SO₂ is supplied from a    vessel that contains the SO₂ gas under pressure until the desired    molar ratio of SO₂ to LiAlCl₄ is obtained. This can be checked by    weighing. The bottle is cooled during supply of the SO₂, whereby the    salt melt dissolves in the SO₂ and a liquid electrolyte according to    the invention is obtained.

An adduct of the Lewis base LiCl and the Lewis acid AlCl₃ is formed bythe described process. The excess LiCl means that the electrolytecontains free LiCl. This prevents formation of free AlCl₃. Generally,independently of the stated example, it is advantageous if theelectrolyte contains free Lewis base in addition to the Lewis acid/Lewisbase adduct. In other words, the mole ratio of the sum of the free Lewisbase and the Lewis base contained in the Lewis acid/Lewis base adduct tothe Lewis acid contained in the Lewis acid/Lewis base adduct should begreater than 1.

The invention claimed is:
 1. An electrolyte for an electrochemicalbattery cell, containing sulfur dioxide and a conductive salt, whereinthe molar concentration of hydroxide groups in the electrolyte is atmost 50 mmol per liter, and wherein the molar concentration ofchlorosulfonate groups in the electrolyte is at most 350 mmol per liter.2. The electrolyte according to claim 1, wherein the molar concentrationof hydroxide groups in the electrolyte is at most 45 mmol per liter. 3.The electrolyte according to claim 1, wherein the molar concentration ofchlorosulfonate groups in the electrolyte is at most 250 mmol per liter.4. The electrolyte according to claim 1, wherein the electrolytecontains at least 1 mol SO₂.
 5. The electrolyte according to claim 1,wherein the conductive salt is a Lewis acid/Lewis base adduct andwherein the electrolyte contains free Lewis base.
 6. The electrolyteaccording to claim 1, wherein the conductive salt is an aluminate,halogenide, oxalate, borate, phosphate, arsenate or gallate of an alkalimetal.
 7. The electrolyte according to claim 6, wherein the conductivesalt is lithiumtetrachloroaluminate.
 8. The electrolyte according toclaim 1, wherein it contains, relative to the mole number of all saltsdissolved in the electrolyte, at most 30 mol % of dissolved salt havinga cation differing from the cation of the active metal.
 9. Anelectrochemical battery cell containing an electrolyte according toclaim 1, a positive electrode and a negative electrode.
 10. The batterycell according to claim 9, wherein the active metal is an alkali metal,an alkaline earth metal, a metal of group 12 of the periodic table oraluminum.
 11. The battery cell according to claim 10, wherein the activemetal is lithium, sodium, calcium, zinc or aluminum.
 12. The batterycell according to claim 9, wherein the negative electrode is aninsertion electrode.
 13. The battery cell according to claim 12, whereinthe negative electrode contains carbon.
 14. The battery cell accordingto claim 9, wherein the positive electrode contains a metal compound.15. The battery cell according to claim 14, wherein the positiveelectrode contains an intercalation compound.
 16. The battery cellaccording to claim 15, wherein the positive electrode contains lithiumiron phosphate.